Sunday, 16 October 2016

Synthesis of graft copolymers onto starch and its semiconducting properties

Published Date2016, Vol.6:538–542,doi:10.1016/j.rinp.2016.08.010Open Access, Creative Commons licenseAuthor Nevin Çankaya,,Department of Chemistry, Faculty of Science and Arts, Usak University, TurkeyReceived 23 May 2016. Accepted 5 August 2016. Available online 11 August 2016.AbstractLiterature review has revealed that, although there are studies about grafting on natural polymers, especially on starch, few of them are about electrical properties of graft polymers. Starch methacrylate (St.met) was obtained by esterification of OH groups on natural starch polymer for this purpose. Grafting of synthesized N-cyclohexyl acrylamide (NCA) and commercial methyl methacrylate (MMA) monomers with St.met was done by free radical polymerization method. The graft copolymers were characterized with FT-IR spectra, thermal and elemental analysis. Thermal stabilities of the graft copolymers were determined by TGA (thermo gravimetric analysis) method and thermal stability of the copolymers is decreased via grafting. The electrical conductivity of the polymers was measured as a function of temperature and it has been observed that electrical conductivity increases with increasing temperature. The absorbance and transmittance versus wavelength of the polymers have been measured.Abbreviations

St.met, starch methacrylate

NCA, N-cyclohexyl acrylamide

MMA, methyl methacrylate

TGA, thermogravimetric analysis (known)

FT-IR spectra, Fourier transform infrared spectra (known)

Keywords

Starch

Graft copolymer

Semiconducting

Thermal stability

Starch methacrylate

Introduction

Starch is abundant in nature and is one of the most important carbohydrates. Biodegradability of starch has attracted attention in the scientific and industrial fields, and there are many uses. However, raw starch has some defects that limit its industrial applications; therefore modifications are needed. Grafting is an important technique for modifying properties of polymers. Chemical modification of starch via graft copolymerization makes starch and synthetic polymer bind together rather than existing merely as a physical mixture[1]. So that, properties of the raw starch have been developed with synthetic monomers and polymers, and semi-synthetic polymers were obtained, increasing application areas and attracting attention of researchers.

Mixture of two different types of polysaccharides glucan (amylose and amylopectin) yields to many different starch structures with various molecular masses. Complete hydrolysis of each component occurs with D-glucose. Water-soluble component of starch is amylose, while amylopectin is water insoluble. Amylose α-(1–4) linked with D-glucose units are composed of very long chains. It shows a broad distribution of molecular weight from 104 to 106 Da, and average degree of polymerization is 1800. The molecular weight of amylopectin one of the most abundant components of the structure of the natural polymer, ranges from 106 to 108 Da. Amylopectin is fairly tight, this is a result of branching due to its molecular weight[2].

Natural and chemically-modified starches can adopt a variety of crystalline structures.

The preferred crystalline structure of a starch sample depends on both the amylopectin structure and amylose/amylopectin ratio. Consequently, the crystalline structure observed is highly dependent on the biological origin of the starch granule [3].

Starch has low solubility in water, low shear stress resistance, thermal decomposition, high retro gradation, and syneresis [4]. In order to extend their applications, functional groups can be introduced into starches by a number of chemical or physical modifications. This may result in improved or specific properties of the starches. A step in chemical modification of starch is the reaction of the hydroxyl groups on the anhydro glucose unit (AGU) [5], thereby producing various starch derivatives.

Semiconducting polymers are now under spotlight as promising materials for the development of optoelectronic devices like light-emitting diodes, photovoltaic cells, and nonlinear optical systems [6], [7], [8], [9], [10], [11] and [12]. It is evaluated that the electrical and optical properties of cellulose and starch can be changed with grafted monomers. Thus, the cellulose/starch can be converted to a semiconductor copolymer.

Similar to the other natural polymer grafting studies by the author and team, this study is about grafting on starch and its electrical and thermal properties. There are many reports upon the synthesis, characterization, and properties of starch graft copolymers; but less attention has been paid to the physical behavior of starch. Thus, the present study describes the esterification of a part of OH starch groups with methacryloyl chloride, and graft copolymerization of starch with NCA and MMA monomers. Thermal stabilities, electric and optical behaviors of the graft copolymers were investigated as well as characterization.

Experimental

Materials

In this study, a uniform powdered potato starch obtained from Sıgma–Aldrıch that contained about 78% amylopectin/22% amylose was used. To increase the surface area, particle size was reduced to 1000 nm. Potassium tert-butoxide was used to break OH bond in starch, and methacryloyl chloride was grafted to the broken bond. Cyclohexylamine (Fluka) was freshly distilled under vacuum prior to use and, N-cyclohexyl acrylamide (NCA) monomer was synthesized by the method from the literature in our laboratory [13], [14] and [15]. Commercial as purchased Methyl methacrylate (MMA) monomer was washed with base solution in order to extract the polymerization inhibitor, and then pure water, dried, and distilled under vacuum before being used. All the other solvents in the process were reagent grade and required no further purification.

Instrumental measurements

FT-IR spectrum was recorded with a Perkin Elmer Spectrum One FT-IR spectrophotometer on solid samples as KBr pellets. Thermal analysis was undertaken with a Shimadzu TGA-50 thermobalance at a heating rate of 10 °C min−1 in a nitrogen flow of 10 ml min−1, while a Leco CHNS-932 was used for elemental analysis. Absorbance and transmittance spectra of the copolymers were measured using a Shimadzu 3600 UV-VIR-NIR spectrophotometer. The electrical conductivity measurements were performed using Keithley 6517A electrometer.

Synthesis of starch methacrylate and graft copolymers

Raw starch has been swollen with acetonitrile as solvent for 1 night. In order to separate H in the primary OH group in starch, it has been reacted with potassium tert-butoxide, which is a strong base, and mixed at room temperature. In order to separate K+ ion, methacryloyl chloride (diluted in acetonitrile) has been added dropwise and the reaction mixture has been refluxed for 24 h. Later, filtered and in order to eliminate precipitations, salts produced and impurities produced by the reaction, it has been rinsed by water, acetonitrile, ethanol, acetone and diethyl ether [14], [15] and [16]. Obtained St.met has been brought to nano-size by using a proper sieve. In order to turn St.met, which is a monomer bonded semi-synthetic natural polymer into graft polymer, synthesized NCA and commercial MMA monomers have been used.

1 g of St.met and 5 g of each monomer NCA or MMA in 50 ml acetonitrile and 1% by mole (0.05 g) AIBN as a free radical initiator were added into polymerization tube, argon was passed, and sealed. The monomer was allowed to graft onto starch at 70 °C for 24 h in the polymerization tube. The reaction that is thought to have happened is shown in Scheme 1. Grafted copolymers were filtered and thoroughly washed with its solvent acetonitrile, N,N-dimethylformamide, chloroform, acetone, and diethyl ether to get rid of oligomers and homopolymers formed as by-products in the reaction [14], [15] and [16]. All grafted copolymers were dried under vacuum.

Scheme. 1. Synthesis of starch methacrylate.

Results and discussion

Grafting of the starch and its characterization

St.met was grafted with NCA and MMA in the presence of 2,2’-azobisisobutyronitrile (AIBN) as free radical polymerization initiator. FT-IR spectra of Starch, St.met, the graft copolymers of starch with NCA and MMA that is St.met-g-NCA and St.met-g-MMA are shown in Fig. 1. FT-IR results of all starch containing polymers show OH peaks at, 3500–3200 cm−1 region clearly. The most important evidence proving the grafting of methacrylate groups onto starch, and therefore forming St.met is the observed ester (–CO) at peaks 1740 cm−1. The presence of a band at 1650 cm−1 (–CO in the amide group) for St.met-g-NCA, and a band at 1730 cm−1 (–CO increase in the ester group) for St.met-g-MMA are the most important evidences of the grafting.

The elemental analyses results of all the polymers are given in Table 1 together with Weight fractions (t) of the monomers in the graft polymers and number (n) of monomer unit per methacrylate group in the graft copolymers. The substitution degree (%) of starch is calculated as 16.3% by mole from the percentage of carbon [14], [15] and [16]. This result shows that substitution on the groups –CH2OH in starch was achieved. Weight fractions (t) of each monomer on grafted starch are calculated from the following:

where Cg, Ci and Cm indicate the percentages of carbon by weight of grafted starch, substituted starch and the monomer are used in the grafting, respectively. The ratio of grafted monomer to methacrylate groups on starch, n, is calculated using the following formula:

In this formula; Mm indicates the molar mass of the monomers, and Mav shows the average molar mass of a starch unit calculation with a substitution degree. The substitution degree (n) in the glucose units of starch was calculated as 16.3% by mole from the percentage of carbon as in the literature. The t and n values found from this contact are given in Table 1. Monomer grafting on starch was of low degree (tNCA= 0.06 and tMMA = 0.16 as weight fraction; nNCA = 0.41 and nMMA = 1.13 as the ratio of grafted vinyl segments to methacrylate groups on starch), because of low St.met binding. Polar monomers such as NCA and MMA may interact with polar groups of starch and these interactions provide these monomers to inhibit reactive centers of starch. It can be seen that, because of the substitution degree in glucose units of starch by mole is 16.3, grafting degree of monomers on starch is not high [14], [15]and [16].

Thermogravimetric analysis (TGA)

It has been shown in Fig. 2 that the initial decomposition temperature belongs to raw starch, followed by St.met and its graft copolymers and has been summarized in the Table 2. Heat stability of unprocessed, raw-natural polymers is higher than semi-synthesized natural polymers. This is an expected result and is in accordance with the literature. It has been seen that the thermal stability of the all the grafting polymers have decreased compared to original raw starch.

Electrical conductivity and optical properties of the copolymers

The electrical conductivity of the polymers is measured as a function of temperature. Fig. 3 shows the plots of σ versus T of the polymers; the electrical conductivity increases with increasing temperature. The conductivity of starch is increased with the methacrylate functional group. St.met-g-NCA exhibits the highest conductivity at 500 K. St.met-g-MMA did not exhibit semiconducting behavior, i.e. it exhibited insulating behavior. Thus, it does not have electrical and optical properties of St.met-g-MMA. The conductivity deviated from the linearity with increase of temperature, and after 350 K; it decreases and then, increases again. This behavior is related to the chemical structure of these polymers. For the semiconducting behavior, the delocalization of electrons should take place in polymer chains. It is evaluated that the chemical structure permits the electron delocalization and in turn, the polymers exhibit semiconducting behavior. In cyclohexene, there is only one π bond and so the electrons in that π bond are localized between those two carbon atoms. For this reason, St.met-g-NCA has semiconducting properties. This confirms that an interaction is taking place between starch and monomers.

Fig. 3. Plots of σ versus T for the polymers.

Absorbance of the polymers increases with decreasing wavelength. When the absorbance of the polymers is compared, the St.met polymer exhibits the highest absorbance in 350 K, whereas natural raw starch polymer exhibits the lowest absorbance. This means that the St.met polymer has the highest electrical conductivity, as shown in Fig. 3. The transmittance of the polymers decreases with decreasing wavelength. The transmittance of the St.met polymer suddenly drops at lower wavelength. This indicates that this polymer has a sharp absorption edge. Also, the St.met-g-NCA polymer exhibited similar behavior. The transmittance spectra of the polymers indicate that the St.met polymer has the highest transparency between 300 and 900 nm. The absorbance and transmittance versus wavelength of the polymers is measured; its plots are shown in Fig. 4 and Fig. 5. The absorbance and transmittance of the polymers change with substitution and grafting [17] and [18]. We can also see the optical band gap (Eg) of the polymers from the first decrease of transmittance spectra of the polymers. In this method [19], the optical band gap among polymers was obtained from the first decrease (at 358 nm) of the only St.met polymer and found to be 3.463 eV.

Fig. 4. Plots of absorbance versus wavelength for the polymers.

Fig. 5. Plots of transmittance versus wavelength for the polymers.

To estimate the polymers, the first derivative (dT/dλ) of the optical transmittance of the polymers is computed [19]. Fig. 6 shows the curves of dT/dλ versus wavelength of the polymers. Then, the absorption band edge (Eg-Abs) of the only St.met polymer can be obtained from the maximum peak position (355 nm) and was found to be 3.493 eV.

Fig. 6. Curves of dT/dλ versus wavelength of the polymers.

Obtained results show that the obtained optical band gap value (3.463 eV) from the first derivative (dT/dλ) of the optical transmittance of the St.met polymer is close to value (3.463 eV) of the absorption band edge of the St.met polymer.

Conclusions

In this study firstly, St.met was prepared by esterification of primary –OH group of starch with methacryloyl chloride in 16.3% yield by mole. St.met was grafted with NCA and MMA monomers via free radical polymerization AIBN as initiator in acetonitrile. The graft copolymers were characterized by FT-IR spectra, thermal and elemental analysis. Thermal stabilities of substituted starch and the graft copolymers were determined by the TGA method and compared with each other. Thermal stability of the copolymers has decreased with grafting. The electrical conductivity of the polymers was measured as a function of temperature and electrical conductivity increased with increasing temperature. As a result, the studied polymers exhibit semiconducting behavior, except St.met-g-MMA. The absorbance and transmittance versus wavelength of the polymers change with substitution and grafting. Obtained optical properties such as optical band gap and absorption band edge show that these polymers exhibit semiconductor behavior and can be used in fabrication of optoelectronic devices such as diodes and metal–semiconductor diodes.